The basic concepts of healthy eating are little-changed from your great-grandmother’s time: Consume a variety of foods, maintain ideal weight and avoid too much salt, sugar and alcohol. Nutrition wars rage over the details, however, and scientists keep changing their advice. Why isn’t it simpler figuring out how to eat healthily? For one thing, nutrition science is relatively young and inherently unreliable. For another, there’s lots of money at stake, leading businesses to argue that what they offer is good for consumers, even if it isn’t.

A massive international study published in 2017 added to evidence undermining concerns about fat consumption. It found that people with the highest intake of total fat had the lowest risk for premature death. The previous year, in its twice-a-decade dietary guidelines, the US government made two notable reversals, dropping its previous advice to restrict dietary cholesterol and total fat consumption. The panel of scientists that drafted the guidelines wrote that a healthy diet is “lower in red and processed meat,” which have been associated with elevated death rates from cancer and heart disease. After strong objections from the meat industry, that caution was dropped. In the final document, men and teenage boys were advised to eat less protein by decreasing consumption of meat, poultry and eggs.

Critics of the complicated US guidelines — the 2016 edition is 204 pages long — argue that they have at times contributed to worsening health. For instance, warnings that the fat in butter and cream produced heart attacks prompted many consumers to switch to margarine and non-dairy creamer, which contain the transfat scientists now think is more hazardous. Admonitions to avoid fat altogether led Americans to eat significantly more carbohydrates, which helped drive the country’s current obesity and Type 2 diabetes epidemics. The 2016 guidelines won praise for focusing less on individual nutrients than on dietary patterns, better reflecting how people actually eat and accounting for the fact that foods interact with each other. Still, detractors note that while the text mentions specific foods that are recommended, when it comes to things to avoid, it reverts to referencing nutrients (added sugars, saturated fats, sodium) rather than items that contain these things (soda, meat and junk food .